Support publishing to personal pages sites in quickstart

Previously pelican-quickstart would assume that the site it created for GitHub Pages should be published to the gh-pages branch. This is correct for project pages, but not correct for personal pages. Personal pages, which live in a
user's special username.github.io repository, are instead deployed to the master branch. This means that if you did pelican-quickstart and tried to publish your new personal site with make github you'd see nothing (or whatever old pages site you had floating around in master).

ghp-import already supports publishing to different branches, so publishing to the correct branch is just a matter of correct configuration and updating the Makefile to pass the branch along to ghp-import. pelican-quickstart now
asks if the user wants to publish to GitHub Pages, and if so, asks if this is a personal page and chooses the correct branch appropriately. I preferred this approach to prompting for an arbitrary branch because I felt that choosing the branch would feel more intimidating to someone using pelican-quickstart for the first time.

This essentially ports changes I made to my personal pages site at jculpon@82cae477a9e8712b90654f6432464369ebcc7ae5. 

I later made further changes to my Makefile to have the commit message reference the commit on the source branch that was used to build the site. If there's interest, I can pull those over too, but I figured that that's a more personal change that most people won't be interested in.